Listeria

• A rare bacterial infection caused by L. monocytogenes which can cause severe disease
• Exposure is via consumption of contaminated foods
• The main presentations are with meningitis/meningoencephalitis, or bloodstream infection, which may be preceded by a diarrhoeal illness (in ~1/4 of cases).
• Generally requires some form of immune system deficit to cause infection e.g. immunosuppression, older age, pregnancy, neonates.
• Can cause a self-limited febrile gastroenteritis in immune competent people.

• Typically associated with soft cheeses, deli foods, but can contaminate quite a wide range of foods.
• Can survive in harsh environmental conditions and replicate at refrigerator temperature. This is why foods that aren’t cooked immediately prior to consumption or that have extended refrigerated shelf lives after processing are often higher risk.

Testing should be undertaken in people with risk factors for infection who present with:

• Systemic febrile illnesses without clear alternative source, especially if preceded by diarrhoeal illness.
• Meningitis or meningoencephalitis

Request blood cultures +/- CSF culture

• Systemic febrile illnesses: 2-3 sets of blood cultures, ideally prior to antibiotics
  • Excellent specificity – a positive blood culture confirms the diagnosis
  • Uncertain sensitivity – due to the lack of a gold standard comparison it is difficult to estimate, however taking 2-3 sets of cultures will optimise sensitivity

• Meningitis or meningoencephalitis: Gram stain and culture of CSF
  • Unlike other forms of bacterial meningitis, a lymphocyte predominance may be seen in the CSF, and the white cell count may only be modestly elevated
  • CSF culture
    • Excellent specificity – positive result confirms the diagnosis
    • Good sensitivity – negative result makes diagnosis unlikely
  • PCR on CSF is also becoming increasingly available.
    • Data are still accumulating in clinical practise, however PCR may have:
      • Slightly better sensitivity than culture, especially if prior effective antibiotic exposure
      • Similar specificity to culture
    • We don’t currently recommend PCR as the standard test for Listeria, however if there is high clinical suspicion then this should be discussed with a clinical microbiologist

• Stool testing
  • There are no reliable protocols for stool testing for Listeria, and the clinical significance of finding it in stool is uncertain, so testing is not offered or recommended.
• Additional neonatal testing
  • This should be done under specialist guidance.
  • In the situation of a neonate born to a mother with Listeria infection during pregnancy, the placenta should usually be examined and cultured. It is important to note the maternal history of Listeria on the request form.

Listeriosis is a notifiable disease.